there are two forces applied to each side of the book. so why is it, than in the free body diagram, there is only 1 force (of 6N). shouldn't you have two forces pointing away from one another, one with 6N (pointing right) and the other with -6N (pointing left), which would cancel out?

Edit: this is the explanation in the book.Consider the free-body diagram below. The force of the fingers on the book is the reaction force to the normal force of the book on the fingers, so is exactly equal and opposite the normal force on the fingers.

Well, you are right, there should be two forces F=6N equal and opposite on each side of the book, but in this case the reaction n in the figure would be zero. What happens here is that it is considered that one of the hands is applying the force F to one side of the book, and the other hand creates the reaction n which is equal in module (n= 6N) and opposite to the force F. Either way to draw the forces leads to the same result: the net horizontal force applied to the book is zero.

The explanation in the book (for drawing only one force F and one reaction n) states that since the two forces applied on the left and on the right side are equal and opposite it is natural (normal) to consider that one is the force and the other its reaction.

I repeat, there is nothing wrong in drawing two forces F equal and opposite on each side of the book, but in this case the reaction n would miss. The third principle of physics (which states that every applied force has an equal and opposing force as reaction) is not broken either way.